Generally, a workpiece is secured to a workholder to prevent movement of the workpiece during subsequent machining, milling, or other finishing operations. Clamping or other such mechanical means are well known in the art for attaching a workpiece to a workholder. However, delicate items, such as an airfoil blade, comprising essentially a thin metal plate having critically contoured surfaces on opposite sides, could be damaged by such mechanical attaching means. Since precise machining is required to produce an airfoil blade having aerodynamically shaped surfaces, precise fixturing is required to hold such a blade during the finishing operation. Movement or improper positioning of the workpiece could easily result in damage to the critical surfaces.
In commonly assigned U.S. Pat. No. 3,586,559 to Shepard et al, a method is disclosed for applying a melted wax-like thermoplastic composition to a workpiece and workholder, which, after cooling, forms an interlayer which bonds the workpiece and workholder together. The wax-like composition and workholder are heated to melt the composition before application and then again after finishing to release the workpiece from the workholder. Generally, such a workholder is constructed with a plurality of internal heating elements imbedded beneath the bonding surface, with the elements capable of heating the workholder to melt the bonding composition. The processing steps include heating the workholder, applying the bonding composition to the workholder, positioning the workpiece on the workholder and cooling the assembly. After finishing, the assembly is heated to melt the bonding composition and thereby free the workpiece. While such a process has been used successfully on many items, there are several limitations.
To insure uniform heating of the workholder bonding surface, a plurality of uniformly disposed electrical heating elements are imbedded under the workholder bonding surface and connected to a power source by a plurality of interconnecting wires. Generally, a workholder having imbedded heating elements is very costly and difficult to construct. This is particularly true for workholders having surfaces which conform to workpieces of complex shape or which require a large bonding surface. In addition, differences in the rate of thermal expansion between the elements, wires and workholder can cause fatigue after a number of heating cycles, resulting in individual element failure. Such failures cause cold spots on the bonding surface, slowing both the bonding and release of a workpiece from the workholder, thereby lengthening processing time.
Another problem involves premature release of the workpiece from the workholder. Generally, a finishing operation, such as milling or grinding, may generate sufficient frictional heat to cause a release of the workpiece from the workholder during finishing. Should this occur, the article may be rendered unusable due to inaccurate machining. The use of thermosetting compositions, which have a higher thermal resistance than thermoplastic compositions, such as the wax-like composition, reduces the likelihood of frictional heat release. However, since such compositions require an even higher temperature to release the workpiece from the workholder, a greater amount of time and energy are required to fix and release a workpiece during the fixturing operation.